
    function objToSearchString(obj) {
        var str = ''
        for (key in obj) {
            str += `${key}=${obj[key]}&`
        }
        return str.slice(0, str.length - 1)
    }
    function ajax(method, url, params, callback) {
        //1.创建请求对象
        var xhr = new XMLHttpRequest()
        //2.配置请求对象
        xhr.open(method, method == 'get' ? url + "?" + objToSearchString(params) : url)
        //3.设置回调函数
        xhr.onreadystatechange = function () {
            if (xhr.readyState == 4) {
                callback(JSON.parse(xhr.responseText))
            }
        }
        //4.发送请求
        xhr.setRequestHeader('content-type', 'application/x-www-form-urlencoded')
        xhr.send(method == 'get' ? null : objToSearchString(params))
    }
